AMER. ZOOL., 21:5-20 (1981)
Functional-Adaptive Analysis in Evolutionary Classification1
WALTER J. BOCK
Department of Biological Sciences, Columbia University,
New York, New York 10027
'
SYNOPSIS. Higher level systematic study can be divided into two parts, namely: (a) the
formulation of classificatory and of phylogenetic hypotheses about groups and their testing against taxonomic properties of characters; and (b) the formulation of hypotheses
about taxonomic properties of characters and their testing against empirical observations
of these features. The second step—character analysis—is most important because it constitutes the actual objective testing within systematic study and provides the basis for one's
conviction in the validity of a particular classification and/or phylogeny. Yet insufficient
attention has been given to the methods used in the character analysis phase of systematics.
Classifications and phylogenies are historical narrative explanations of the past evolutionary history of organisms. As such they must be based on and tested against the underlying theory of organic evolution. Classifications are not used to test the validity of
mechanisms of evolutionary change; rather the reverse is true. For the construction of a
classification or of a phylogeny, it matters absolutely how the organisms evolved—the
classification and phylogeny depend completely on the accepted mechanisms of evolutionary change, not just on the hypothesis that organic evolution had occurred. Thus, the
methods used in the character analysis phase of systematic study must be based on accepted mechanisms of evolutionary change. If the evolutionary change is adaptive, then
functional and adaptive analyses form an integral part of the character analysis.
Several commonly used methods of character analysis, most importantly that of outgroup comparison, are invalid because they are based on simple circularity, etc. The valid
methods available for testing taxonomic properties, e.g., homology, apomorphy, etc., of
characters are poor tests in that they have low resolving power, i.e., poor ability to distinguish between correct and incorrect answers to individual tests. Additional methods must
be used to establish a degree of confidence in particular answers of valid tests of low
resolving power. A set of valid methods are outlined for testing character hypotheses
about homology, apomorphy-plesiomorphy and synapomorphy, and for establishing the
degree of confidence in the results of individual tests. Functional-adaptive analyses are a
necessary and essential part of the valid methods used to test and/or establish the degree
of confidence in individual results for all hypotheses about taxonomic properties of characters.
INTRODUCTION
1
, „
received far less attention, covers the
methods by which hypotheses about classifications and phylogenies (theoretical scientific statements) are tested against empirical observations
(observational
statements).
This part of systematic study is concerned with character analysis under
which I understand (a) the formulation of
hypotheses about the taxonomic properties of characters, (b) the valid methods by
which these hypotheses are tested against
empirical observations, (c) the establishment of a degree of confidence in outcome
of individual tests, and (d) the use of the
accepted hypotheses about taxonomic
properties {i.e., those that have not been
rejected by the empirical tests) to test the
nas
General discussions in that area of systematics concerned with the construction
of classifications and of phylogenies can be
grouped under two headings. The first,
which has received considerable attention
over the past few decades, deals with theoretical questions on the purposes and nature of biological classification and on the
merits of the diverse approaches. Most of
the recent controversy between cladists
(Hennig, 1965, 1966, 1974) and evolutionary systematics (Mayr, 1969, 1974, 1976),
for example, has dealt with these theoretical questions. The second heading, which
, ,
,
From the Symposium on Functional-Adaptive Anal-
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ysis in Systematics presented at the Annual Meeting of classificatory and the phylogenetic hypoththe American Society of Zoologists, 27-30 December e s e s about groups.
1979, at Tampa, Florida.
T h e goals of this paper are to outline
WALTER J. BOCK
the methods of character analysis used
within evolutionary classification and to inquire into the role of functional-adaptive
analysis (F-A analysis) in these studies.
Herein I am not concerned with the relative merits of the diverse approaches to
classification and no arguments will be given to support my acceptance of the evolutionary approach to classification (see
Bock, 1974, 1977a) as these are beyond the
scope of this symposium.
At the onset, I would like to express two
major tenets of my position on character
analysis in systematics. The first is that
character analysis, not the taxonomic and
phylogenetic conclusions about groups, is
the most important part of any systematic
study. Classificatory and phylogenetic hypotheses about groups are easy to formulate, but gain importance only in proportion to the strength (conviction) of the
supporting character analysis. Second is
that the methods of character analysis and
their validity are independent of the accepted approach to classification. It would
follow that the differences between the
several currently advocated approaches to
classification are smaller than usually argued and appear to be ones of degree
rather than of kind. A purely phenetic approach or a purely cladistic approach to
classification may not be possible because
the methods of character analysis appear
to be common and pertinent to all approaches to classification.
BIOLOGICAL CLASSIFICATION
Biological classification is a heuristic device that permits summary of information
about biological organisms and provides
the basis for comparative studies in biology—thus classification provides the foundation for efficient scientific explanation in
biology (Warburton, 1967; Bock, 1977a).
Classifications are constructed by arranging individual species into groups and
these groups into more inclusive ones, etc.,
using a set of rules derived from underlying theory (e.g., the theory and mechanisms of organic evolution). The final result is a hierarchial scheme of
nonoverlapping, inclusive groups. The
best approach to classification is that one
which generates systems admitting the
largest number of useful testable hypotheses (e.g., theoretical scientific statements)
about organisms.
Although some disagreement exists on
the meaning of scientific theories and h y ^
potheses and on whether biological classifications fall under this heading, it is clear
that they possess a number of attributes of
scientific hypotheses and should be so considered. In particular, a classification offers an explanation for a set of observable
facts and, most importantly, it must be
tested against empirical observations. Classifications, as all scientific hypotheses, are
theory laden statements and after 1859,
biological classifications are founded on
the theory of organic evolution.
To provide this basis for efficient explanation in biology, classifications must be
natural as opposed to artificial or designed
for special purposes. By natural, I mean
that classification conforms to the underlying theory, i.e., organic evolution, and to
all predictions arising from it (Bock,
1977a, pp. 863-864). This concept of natural differs from the sense of natural advocated for classifications by Gilmore
(1940) which is that natural classifications
are based on all attributes of organisms,
i.e., that taxa are recognized on the basis
of maximum concordance of similarities of
all phenotypical attributes of the contained
members. I reject this concept of natural
because it is based on an operational rather
than a theoretical approach to classification (see Hempel, 1965, for a discussion of
the difficulties of operationalism). Under
the concept of natural advocated herein,
no set of phenotypic features can be excluded automatically in the construction of
a classification.
Given that classifications should be natural, then only one classification should be
used for any group of organisms and that
classification should be the one permitting
the maximum number of useful, testable
scientific hypotheses. It is clear that the single best natural classification will not be as
optimal for a particular study as would be
a special classification designed for that
study. However, the results of diverse
studies can be compared directly only if
EVOLUTIONARY CLASSIFICATION
the same natural classification is used for
all studies, and such comparisons are essential if we hope to achieve a unified science of biology.
Little disagreement exists among prese n t day taxonomists, myself included, that
biological classification must be based on
organic evolution. For example, Gaffney
(1979, p. 86) states that acceptance of the
hypothesis that evolution has occurred is
one of the two assumptions required for
the construction of classifications (Gaffney
uses the term "geometry of descent" but
this term equivalent to classification under
the tenets of cladism). It is certainly essential to accept the occurrence of evolution
for the construction and testing of classifications, but although this assumption is
necessary, it is not sufficient. My statement
is in addition to the second assumption—
"that new taxa may be characterized by
new features"—mentioned by Gaffney
(1979, p. 86). Beyond this broad generalization of accepting the occurrence of evolution and hence that "Life has one genealogy (phylogeny)" (Gaffney, 1979, p.
86), are a diversity of conflicting views on
the relationship between mechanisms of
evolutionary change and the construction
of classifications, on the direction of testing
these several types of scientific hypotheses,
etc., all of which bear on the role of functional-adaptive analyses in the testing of
classifications (and of phylogenies) against
empirical observations. These divergent
views must be discussed in detail.
Most important of these divergent views
is the symmetry of testing evolutionary
theories (including mechanisms) and classifications. Are classifications tested against
particular mechanisms of evolutionary
change or are mechanisms of evolutionary
change tested against classifications? Many
taxonomists claim that the latter is true,
e.g., "if classifications (that is, our knowledge of patterns) are to provide an adequate test of theories of evolutionary process" (Platnick, 1979, p. 539) and "It is not
necessary to rely on this hypothesis (or any
other) of evolutionary processes in order
to set up and test phylogenetic hypotheses
by cladistic methods. Although some critics
(e.g., Mayr, 1974) have mistakenly thought
that the method does depend on a speciation model, the error of this viewpoint has
been made clear by a number of authors
(see especially Platnick, 1977f)" (Gaffney,
1979, pp. 86-87, footnote 5). These statements typify the position held by many recent cladists who differ markedly from the
ideas of phylogenetic systematics advanced
by Hennig (1966). These ideas that classifications are used as tests for mechanisms
of evolutionary change are the basis for
the further claims by many taxonomists
(including many cladists) that particular
evolutionary mechanisms ( = processes)
such as allopatric speciation or the process
of adaptation cannot be used in the construction of classifications. To do so, these
workers argue, would be to engage in direct circular reasoning. But the idea that
classifications are tests for evolutionary
mechanisms and all arguments that stem
from this claim are mistaken because they
reverse the proper direction of explanatory flow and of testing. Simply stated,
nomological-deductive (N-D) explanations
are used to test historical narrative (H-N)
explanations, never the reverse {e.g., Nagel, 1965).
Classifications of biological organisms
are based on evolutionary theory and as
such they represent a scientific (e.g., historical narrative, see below) explanation
for a set of observed facts in accordance
with the underlying theory (Bock, 1977a,
1978). The theory of evolution and the
mechanisms of evolutionary changes (e.g.,
processes such as geographical speciation,
adaptive phylogenetic change) are nomological-deductive explanations (Bock and
von Wahlert, 1963; see Nagel, 1961; Hempel, 1965, especially Chapter 12, for a dis^
cussion of nomological-deductive explanations). Because a classification (or a
phylogeny) is an attempt to explain the
evolutionary history of a particular group
of organisms, the explanation must flow
from the set of accepted evolutionary
mechanisms to the classification or the
phylogeny. For the construction of the
classification or the phylogeny of a group,
it makes a great deal of difference if speciation occurs by allopatric separation, by
hybridization followed by chromosome
8
WALTER J. BOCK
doubling or whatever, or if phylogenetic
evolutionary change is by gradual steps
postulated in the synthetic model (e.g.,
Bock, 1979), by large saltations, or by a
pattern of always three forward steps followed by two backward ones. Moreover, as
will be argued below, historical-narrative
explanations (e.g., classifications and phylogenies) of groups of organisms cannot
provide empirical tests for nomologicaldeductive explanations (e.g., mechanisms
of evolutionary change) in biology.
Classifications are used to summarize information about biological organisms in an
efficient way and hence it is proper to regard them as expressions of a particular
arrangement of order in biological organisms. The meaning of this order and its
underlying foundation are most important. Does it exist in nature, ready to be
found and once discovered provide the
foundation for general explanation, or is
the order expressed in classifications a reflection of prior underlying theory? The
frequent statements by many taxonomists,
including cladists, that classifications are
"an extrapolation from hypotheses about
natural order" (Platnick, 1979, p. 537),
that classifications reflect "our knowledge
of patterns" (Platnick, 1979, p. 539), or are
"a natural order of sets and subsets" and
express "natural order" or "nature's hierarchy" (Rosen et al., 1979, p. ix) without
any reference to underlying theory are
vague to the point of being meaningless.
Concepts of a "natural order" and of "nature's hierarchy" existing in biological organisms sound exactly like statements
from the ideas of natural philosophy (Naturphilosophie, ideal typology) used as the
foundation of classification prior to 1859.
This is an essentialist concept of classification (Mayr, 1969, pp. 66-68).
Order does exist in nature and it can be
discovered, but its value for scientific explanations is dependent on the theory underlying the discovery of a particular type
of order. If classifications are to be the basis of scientific explanations about organisms, then the order expressed in their
classification and their phylogeny must be
based on theory which is, of course, organic evolution. The order among biological
organisms expressed in classifications (and
in phylogenies) is a reflection of the evolutionary history of that group of organisms. And this evolutionary history cannot
be understood without reference to the
mechanisms of evolutionary change.
•
It is, therefore, not sufficient to state
that the only hypotheses required for biological classification are "that evolution
has occurred and that new taxa may be
characterized by new features." (Gaffney,
1979, p. 86) and it is wrong to claim that
no reliance is needed on the synthetic theory of evolution or on particular mechanisms of evolutionary change for the construction of classifications (Gaffney, 1979,
p. 86). Nor is it correct to claim that "if
classifications (that is, our knowledge of
patterns) are ever to provide an adequate
test of theories of evolutionary processes,
their construction must be independent of
any particular theory of process" (Platnick,
1979, p. 539). And statements that "The
distinction between plesiomorphic and
apomorphic character states depends not
on the reconstruction of actual evolutionary history, but on the discrimination of
more general from less general characters"
(Platnick, 1979, p. 537) have no meaning,
given the above argument. What is meant
by more general characters and less general characters? On what theory is such a
distinction based? And how can characters
corresponding to these categories be recognized and distinguished when observed
in nature?
To say that classifications are natural if
they correspond to the underlying theory
(e.g., organic evolution) and all resulting
predictions means that the construction of
classifications (and of phylogenies) must
depend on the details of the accepted
mechanisms of evolutionary change. It
matters absolutely for the construction of
classifications and phylogenies whether
one accepts the synthetic theory of evolution or some other theory of evolutionary
change. A necessary and sufficient set of
assumptions on which classifications and
phylogenies are developed must include,
not only the occurrence of evolution, but
the accepted set of mechanisms by which
this evolutionary change has taken place.
EVOLUTIONARY CLASSIFICATION
That is, proper testing of classifications tions. The first three may be grouped
and of phylogenies is dependent absolute- together as nomological-deductive explaly on the use of accepted mechanisms of nations and the last—genetic—is equivaevolutionary change. The theory of organ- lent to historical-narrative explanations.
ic evolution, i.e., that evolution has taken
Nomological-deductive (N-D) explanae and that the observed attributes of tions (e.g., causal law) are in the form, givorganisms are the result of this evolution, en a set of facts (e.g., initial and boundary
and the particular mechanisms (=process- conditions) and a set of laws, both of
es) of evolutionary change, both of specia- which form the explanas or explanatory sention and of phyletic evolution, are tested tence, a particular conclusion or the exagainst empirical observations that are in- planandum follows (Hempel, 1965, pp.
dependent of the classification and phylog- 335-338). A N-D explanation answers the
eny of groups.
question: Why has a particular explananGiven the above argument, then the dum-phenomenon occurred? It does so by
methods by which classifications (and phy- showing that the explanandum resulted
logenies) are tested against empirical ob- from the particular initial and boundary
servations—the character analysis phase of conditions set forth in the set of factual
systematics—must depend on and be con- sentences in accordance with the set of
sistent with the accepted mechanisms of laws invoked (see Hempel, 1965). N-D exevolutionary change. Among other things, planations apply to universals (indefinite
one must have the basis for ascertaining sets of phenomena), do not depend on the
the sequence of the modifications of fea- past history of the objects or phenomena
tures in their evolutionary change, for de- being explained, and their premises are
termining the direction of this sequence, always true. If the explanandum did not
for claiming whether the change is revers- always follow from the conjunction of the
ible or not, etc. Such interpretations de- set of sentences of facts invoked and the
pend on estimating the adaptiveness of set of general laws, then the N-D explafeatures and their possible adaptive evo- nation is not valid (it has been falsified)
lutionary change—therefore the need for and one must search for the basis for the
functional-adaptive analysis in systematics. falsification. Possibly one of the general
laws on which the explanation depends is
not valid or possible the set of facts (initial
EXPLANATION IN SCIENCE
and
boundary conditions) are not correct.
Classifications and phylogenies are sciof N-D explanations include
Examples
entific explanations that must be tested
Newton's
three
laws of motion and the
against empirical observations. Before the
Hardy-Weinberg
law of genetic equilibrimeans of testing classifications and phylogenies can be outlined, the nature of differ- um.
ent explanations in science must be exHistorical-narrative (H-N) explanations
plored.
provide an explanation of the existing atScientific explanations may be grouped tributes of a set of objects at any point in
into two major categories—nomological- time; these explanations depend on the
deductive (N-D) and historical narrative past history of these objects and must use
(H-N)—which are both testable theoretical the pertinent nomological-deductive exscientific statements, regardless of whether planations. The objects being explained by
they are called theories, hypotheses, con- a H-N explanation are not universals, but
cepts, etc. Both types of explanation are are particulars—a finite set having definite
scientific in that they can be falsified and spatial-temporal positions. H-N explanaboth must be tested against empirical ob- tions are always given on a nondeductive
servations. Nagel (1961, pp. 20-26, 1965) basis with the hope of obtaining the most
divides explanation in science into four reasonable and probable explanation for
categories—the deductive model, proba- the objects under study. Two aspects of
bilistic explanations, functional or teleo- H-N explanations must be stressed. First
logical explanations, and genetic explana- is that the explanation is always given on
10
WALTER J. BOCK
a probability basis (Nagel, 1961, p. 26)
which results from the large number, and
often conflicting, nomological generalizations that must be used in the H-N explanation. Second is that any H-N E must depend on the pertinent N-D E and hence
these N-D E must be included in the chain
of argument used in the testing of the HN E.
Although both N-D E and H-N E are
scientific under the criteron of demarcation advocated by Popper in that they can
be falsified and both must be tested against
empirical observations, they differ in many
aspects of how they are expressed, how
they are tested and how they can be used
to test other theoretical statements. Almost
all philosophical analyses apply only to
N-D E and many aspects of these analyses
are not suitable or valid for H-N E. Unfortunately these distinctions between N-D
E and H-N E were not appreciated by
many systematists with considerable resulting confusion in the nature and testing
of explanations in systematics. My following comments will be restricted to H-N E.
Being theoretical scientific statements,
H-N E are available to tests by falsification;
however, it is also possible to test them by
confirmation with the addition of more
and more positive tests. For example, classifications may be tested by the discovery
of more and more homologous features
which are in agreement with the classificatory hypothesis about the group. Thus
testing of H-N E is closely akin, if not identical, to induction in the strict sense. No
objections to this should exist because H-N
E are scientific statements about a finate
number of objects in contradistinction to
N-D E which cover universals. In the testing of H-N E, predictions must be developed using argument chains involving the
pertinent N-D E and a large number of
background assumptions (some of which
are in the nature of initial and boundary
conditions) and then tested directly against
empirical observations. One must proceed
to the empirical observations rapidly
and directly although the route is often a
complicated chain of arguments. The empirical observations and their role as tests
for particular theoretical statements must
be designated clearly.
It is not valid to test a H-N E using parsimony as the criterion (Patterson, 1977, p.
628) as has been advocated by some systematists. Parsimony has been used errcS*
neously to indicate the assumption that the
evolutionary history of a group always involves positing the minimum number of
changes, as commonly used by comparative biochemists (e.g., Fitch, 1977, pp. 214219) for the construction of phylogenetic
trees from data on amino acid sequence of
proteins. Assumption of a minimum number of evolutionary changes may be a reasonable one, but it is a narrative explanation in itself and must be tested somehow
against empirical observations. And even
if tested successfully for one group, the
"parsimony" argument cannot automatically be applied to other groups and it cannot always be used to test other H-N E.
It is not valid to test one H-N E by the
use of a second H-N E of greater generality. This approach can be used in testing
N-D E where a N-D E of greater generality
is used as a test of one of lesser generality
because the premises of N-D E must always
hold true, being deductive explanations.
But H-N E are presented as nondeductive,
probability explanations and have very restricted application. Even if it can be
shown that one H-N E has greater generality and includes a second, less general
H-N E, a demonstration which is difficult
to show, the more general H-N E cannot
be used as a test for the less general one
because it may be one of the exceptions
not covered by the more general explanation. This follows from the fact that HN E are nondeductive probability explanations. This conclusion is an extremely
important one and will be referred to below in the discussion of methods used to
test classifications and phylogenies.
A clear distinction must be made between theoretical scientific statements and
empirical observational statements used to
test them (Nagel, 1979, Chapter 5). Both
are theory laden, but the theories underlying the observational statements used to
test a particular theoretical statement can-
EVOLUTIONARY CLASSIFICATION
not be part of or cannot be directly derived
from the theory being tested. If that were
the case, then the empirical test of the theory would be directly circular; it would not
provide the independent objective empir^al test demanded by the commonly accepted tenets of science. This conclusion
is also extremely important and will be referred to when discussing particular types
of tests of theoretical statements advocated
by some taxonomists.
Within the scope of evolutionary biology, a distinction must be made between
those explanations that are N-D E and
those which are H-N E. The general statement of organic evolution that populations
of organisms may change over time and
the mechanisms of evolutionary change
such as the process of speciation and the
process of adaptive phyletic evolution are
N-D E. By this I mean that the formulation, test and use of these explanations are
of the standard nomological law type. I do
not mean that only one mechanism of speciation exists—several may, but each is
built on the nomological-deductive model.
Nor do I mean that it is always possible to
show that a particular evolutionary mechanism (a N-D explanation) was responsible
in any given example of a historical evolutionary change (a H-N E) because if it
were, then explanations of historical evolutionary changes would be N-D E which
they are not.
Statements about the history of life,
about the evolution or phylogeny of any
group of organisms, about the classification of any group, about the particular attributes of any features of organisms, etc.,
are all historical-narrative explanations
(Bock and von Wahlert, 1963; Bock,
1978). All classifications and all phylogenies, whether purely descriptive or explanatory, are H-N E (Bock, 1978) contrary to
the ascertains of many taxonomists (mainly
cladists) who argue that classifications and
phylogenies are N-D E (Platnick and Gaffney, 1977). H-N E classifications and phylogenies are proper scientific explanations
(Bock, 1974, pp. 381-383, 1977a, p. 854)
contrary to claims of some taxonomists,
but they are not covered by the ideas on
11
N-D E developed by many philosophers of
science. In particular, the ideas on scientific explanation advocated by Popper do
not apply directly to classifications, phylogenies and other H-N E as Popper himself
is quite explicit in excluding H-N E from
his view of science—a position that I do
not accept for two reasons, namely: (a)
H-N E can be falsified and hence fall under the heading of science using the criterion of Popper; and (b) a large percent,
perhaps over 90%, of all work done by scientists deals with H-N E.
FUNCTIONAL AND ADAPTIVE ANALYSES
Because the core of my thesis is that valid tests of classificatory and of phylogenetic hypotheses about groups must include
functional and adaptive analyses (F-A A)
of the taxonomic characters, it is essential
to state exactly what I mean by functional
and adaptive analyses. My comments will
specify only morphological features but
are valid for all taxonomic characters
evolving by standard Darwinian mechanisms. I believe that these comments may
also apply to features which change by cultural modes of evolution, but cannot support this belief at this time. Greater difficulty probably exists for the application of
my comments to features evolving by nonDarwinian evolution should this type of
evolutionary change exist. Functional analysis and adaptive analysis are separate, but
related studies; both of which are required
for proper testing of classificatory and
phylogenetic hypotheses about groups. My
comments are based on my earlier papers
on adaptation (Bock, 1959, 19776, c, 1980;
Bock and von Wahlert, 1965). Further, my
analysis is based on the acceptance of the
reductionistic model of macroevolutionary
change postulated under the synthetic theory of evolution (Bock, 1979).
Under functional analysis, I include all
studies leading to the understanding of
functional properties of features (function
is used throughout in the sense of Bock
and von Wahlert, 1965). This work is dependent on accurate, detailed morphological description of features which must be
done with an understanding of the func-
12
WALTER J. BOCK
tional properties of those features; hence,
feedback must exist between morphological description and functional analysis.
Contrary to the approach used in most
taxonomic studies, the morphological description of taxonomical characters cannot
be done in the absence of proper functional understanding because no guide would
exist to the functionally and adaptively significant aspects of these features. Needless
to say the descriptive and functional studies must be comparative. The pertinent
functional properties must be obtained by
direct observations on the taxonomic features under study or from prior knowledge of form-functional correlations (see
Bock and von Wahlert, 1965) of analogous
features reached in earlier studies. The
first is ideal and should be used whenever
possible. In any case, the functional studies
must be based on direct observations, be
these of the physiological properties of
bones, muscles, tendons, etc., or of the
working of complex systems, such as the
jaw apparatus or of the locomotory system,
etc.
Morphological structures are composed
of materials possessing physical and chemical properties; hence the functional analysis must include the necessary physical
and chemical laws (N-D E). Because many
of the morphological features used as
taxonomic characters are parts of mechanical systems, it is usually necessary, or at
least desirable, to include the appropriate
laws of mechanics, hydraulics, etc. in the
analysis. Although taxonomists have been
reluctant to include these physical laws in
their functional analysis, the use of simple
physical concepts frequently provides great
clarification to otherwise murky functional
and adaptive analyses.
Practicalities of study may limit functional analysis to one feature or one complex of features at a time. Nevertheless,
one must always remember that any organism, be it an existing one or a postulated intermediate, must be an integrated,
functional whole (Dullemeijer, 1974). In
addition to functional analyses of individual features and structural complexes, it is
necessary to determine whether the features postulated to exist in the same or-
ganism can function together. And one
must show that the suggested sequence of
evolutionary changes of individual features in a phyletic lineage always results in
functional organisms which are able to survive in the indicated environment. T h ^
essential facet of functional analysis has
been overlooked by almost all taxonomists
who limit themselves, at most, to the formulation of transformation series of individual features. In a long series of papers,
Gutmann (e.g., 1972, 1975, 1977) demonstrated the need to establish phylogenetic sequences of functional organisms in
addition to transformation series of individual features as the foundation of any
systematic study.
My ideas on the definition, recognition
and origin of biological adaptations have
been given in earlier papers (Bock, 1959,
1977&, c, 1980; Bock and von Wahlert,
1965) and need not be repeated here.
Study of the form and the function of features in the laboratory, no matter how
thoroughly done, is not a sufficient basis
on which to determine their adaptive significance. It is essential to do field work on
the biological role of the features, on the
environmental demands on the organism
and on the plausible selection forces. Only
then, is it possible to determine the relationship between the feature and the" selection force, to judge whether an adaption exists, and to estimate its degree of
goodness. The total work required to ascertain adaptations is far more difficult
than usually believed, and errors can be
made in the establishment of an adaptation even when the analysis is complete
and thorough. Far fewer features have
been shown to be adaptations in any detail
and with any assurance than believed by
most biologists. Yet many examples can be
cited in which the adaptiveness of the features are well supported. These include
the heterozygous Hb A /Hb s condition in
humans (Allison, 1956), the color and
banding pattern of Cepaea nemoralis (Cain
and Sheppard, 1954; Ford, 1971, pp.178203), industrial melanism in several species
of moths (Kettlewell, 1959, 1973), avoidance behavior of insect prey to sonar calls
of bats (Roder and Treat, 1961; Fullard,
EVOLUTIONARY CLASSIFICATION
et al, 1979; Miller and Olesen, 1979; Olesen and Miller, 1979; Simmons et al.,
1979), thickness of plumage and peltage in
Arctic birds and mammals (Scholander,
1955), the structure of the sublingual
^ouch in nutcrackers, Nucifraga (Bock et
al., 1973), the large mucous gland in the
gray jays, Perisoreus (Bock, 1961; Dow,
1965), the length of protractor and retractor tongue in woodpeckers, hummingbirds, other nectar-feeding passerine birds
(personal observation), and the large adductor jaw muscles with many short fibers
in seed-eating finches (personal observation). It is even more difficult to demonstrate the process of actual adaptive phyletic evolutionary change because of the
large time factor involved, but these have
been shown in the change of resistance of
insects to DDT, the development of resistance of bacteria to penicillin and other
antibiotics, and the change in predominance of the dark color morph in moths
(industrial melanism) with increased pollution and its reversal as pollution lessened
(Bishop and Cook, 1975).
It is reasonable to argue that the demonstration of adaptions or of adaptive evolutionary change is not an easy task and
that far too many adaptive conclusions
have been offered without sufficient support. And it is always possible to query or
reject particular adaptive interpretations.
But I reject as extreme those statements
that claim adaptations and adaptive evolutionary changes have not been demonstrated or, if so, only with inadequate support.
Investigation of the adaptive significance of features is only possible on a
foundation of thorough functional studies
because the concept of adaptation is linked
to both the form and the function of features (Bock and von Wahlert, 1965). The
relative lack of functional analyses in morphology until about three decades ago had
been the major deterrent to the development of proper adaptive analysis. Beginning in the early 1950s greater interest developed in functional morphology with the
availability of new techniques and equipment. This field has grown until today it
has become the most active, and is still the
13
most rapidly expanding, part of morphology. In the absence of a vigorous experimental functional morphology, ideas in
evolutionary morphology would have stagnated and it would not have been possible
to develop and apply the concepts of functional and adaptive analyses to systematic
studies. The relative lack of application of
functional and adaptive analyses in the
construction of classifications and phylogenies is a reflection of the newness of these
ideas, not of their importance for systematics.
CLASSIFICATION AND PHYLOGENY
The classification and the phylogeny of
a group of organisms are both historical
narrative explanations and, as such, are
scientific hypotheses to be tested under the
tenets of H-N E discussed above. Under
the concepts of evolutionary classification,
the classification and the phylogeny of a
group are not equivalent or redundant. It
is necessary to formulate separate classificatory hypotheses and phylogenetic hypotheses of the groups under consideration and to test each type of hypothesis
separately (Bock, 1977a).
Classificatory hypotheses about groups
express the evolutionary relationships of
the constituent members within a formal
hierarchial system under the conventions
accepted for evolutionary classification.
The formal classification is a Linnean hierarchy and the rules for recognizing the
groups would be those that maximize simultaneously the degree of evolutionary
change and the sequence of phylogenetic
events. The groups are taxa, each of which
is monotypic in the broad sense. These
taxa are arranged in an inclusive, nonoverlapping hierarchy.
Phylogenetic hypotheses about groups
express the pattern of phylogenetic
branching of the constituent members
within a formal phylogenetic diagram under the conventions accepted for these diagrams. The formal phylogeny is a dendrogram (phylogenetic tree) with the branches
being dichotomous forks as advocated by
Hennig (1966). The groups are phyla (singular, phylon; Bock, 1977a, p. 877) which
are closed descendent groups. Each phy-
14
WALTER J. BOCK
Ion must be holophyletic (Ashlock, 1971),
a group that includes the ancestral species
and all descendent species. Phylogenetic
hypotheses about groups can express sister-group relationships and ancestral-descendent relationships.
Both types of hypotheses about groups
are tested against, what I will term, taxonomic properties of features which in turn
must be tested against empirical observations—the character analysis phase of systematic study. By taxonomic properties of
features (or characters), I mean all phylogenetic attributes of features, such as homology, plesiomorphy and apomorphy,
synapomorphy, etc., which stem from the
evolutionary history of the group. Tests of
classificatory hypotheses about groups and
of phylogenetic hypotheses about groups
overlap to some extent, but are not identical.
CHARACTER ANALYSIS
Under the heading of character analysis,
I include the formulation and testing of
hypotheses about taxonomic properties of
characters (Bock, 1977a, pp. 879-891) and
the use of these taxonomic properties of
characters to test classificatory and phylogenetic hypotheses about groups. Character analysis is the heart of any systematic
study. The role of F-A A in character analysis is the core of this symposium.
The important aspect of character analysis is not the formulation of hypotheses
about taxonomic properties of features,
but how these character hypotheses are
tested against empirical observations. A
clear distinction must be made between
valid (proper) and invalid tests and between good and poor tests. By a valid test,
I mean one that relates predictions arising
from the hypothesis to empirical observations according to the stipulations of the
theory. For H-N E, the stipulations are established by the underlying N-D E. By a
good test, I mean a valid test that has a
high ability to distinguish between correct
and incorrect hypotheses—that is, one that
has a high resolving power between correct and incorrect answers. Most, if not all,
valid tests available to systematists to test
character hypotheses are poor ones that
have very low resolving power to distinguish between a correct and an incorrect
hypothesis. Many workers have confused
poor, but valid tests with invalid ones and
have incorrectly discarded these poor, but
valid tests.
^
Because of the poor resolving power of
the valid tests available to systematics for
the character analysis, additional study is
required to establish a degree of confidence in the results of the test. This additional study does not increase the validity
of the empirical tests of the character hypotheses or increase the correctness of the
result, but provides an estimate of the
probability of correctness of the result of
the test—it is a measure of the degree of
confidence in a particular valid test. Most
of the consideration given by taxonomists
to criteria for homology (e.g., Rieger and
Tyler, 1979) are methods to establish the
probability of correctness of the test's outcome. Because these supplementary studies only increase or decrease the degree of
confidence in the correctness of the hypotheses, they cannot be regarded as additional valid tests for the hypothesis.
Assessment of the multitude of tests for
character hypotheses used by taxonomists
to separate the valid and invalid ones is a
difficult task. Only a small number of valid
tests appears to be available. Before examining these valid tests, I would like to
comment briefly on a few invalid tests used
commonly by taxonomists. These are:
a) Any tests that employ parsimony as
the criterion. These tests are either a misuse of the concept of parsimony (Bock,
1977a, pp. 859-860) or a misapplication of
this term to designate a particular assumption, i.e., minimum number of evolutionary steps to reach a particular character
state. It is frequently unclear whether parsimony is being applied as the criterion to
test hypotheses about groups or to test
hypotheses about characters.
b) Any tests that employ better internal
consistency or better internal logic of the
group hypothesis or of the character hypothesis as the criterion. Internal consistency or logic are desirable and essential
properties of theoretical statements, but
they are not empirical tests.
EVOLUTIONARY CLASSIFICATION
c) Any tests that employ parallelism of
the pattern of observed changes in ontogeny with the pattern of postulated changes
in phylogeny as the criterion (i.e., ontogeny recapitulates phylogeny). No N-D E
"•has been demonstrated in the relationship
between these two patterns of change
which is needed before ontogenetical
changes can serve as a test for postulated
phylogenetical changes.
d) Any tests that employ the distribution of character states in taxonomic
groups as the criterion. These are among
the most commonly used by all taxonomists and include tests such as the consistency of the character in the taxon as the
criterion for conservative or good taxonomic characters and the method of outgroup comparison. The latter is most important because it is the major method
used by cladists to test hypotheses about
synapomorphies—it is hence the basic
method used to test hypotheses about
groups in cladistics. These methods depend on examination of the distribution of
character states in the taxon under study
and/or in the most closely related taxa—
the so-called out-groups—to test hypotheses about the taxonomic properties of
characters. And these taxonomic properties of characters are then used to test classificatory and phylogenetic hypotheses
about the same groups. For example, in
the out-group method, a feature (character state) found only in the group under
study or possibly in that group and neighboring groups is judged to be synapomorphous in the groups possessing it. If the
feature is found in a wide range of outgroups, it is judged to be symplesiomorphous in those groups possessing it. The
synapomorphous features are then used to
test the phylogenetic hypotheses about
these groups. The argument used to support the method of out-group comparison
is that any hypotheses about groups of
higher categorical rank (more inclusive
groups) are more general ones and hence
can be used to test hypotheses about
groups of lower categorical rank (less inclusive groups). This appears to be the basis for Platnick's (1979, p. 537) statement
that the distinction between plesiomorphic
15
and apomorphic character states depends
on the discrimination of more general
from less general characters. Unfortunately these methods are not valid because
they are directly circular, suffering from
two fatal flaws. The first is that the hypotheses involved, both the group and the
character hypotheses, are H-N E which are
nondeductive, probabilistic explanations,
and hence a more inclusive one (even if it
could be shown to be a more inclusive explanation) cannot serve as a valid test for
a contained one. Second is that part of the
theory used in the observational statements stems directly from the theory being
described in the theoretical statements
(note that the taxonomic groups described
in the group hypotheses are used in the empirical observations serving to test the
character hypotheses). Hence the observational statement is dependent upon the
theoretical statement that is being tested
which renders the test directly circular
(Nagel, 1979, Chapter 2) and hence invalid. It should be emphasized again that although both theoretical and observational
statements are theory-laden, observational
statements can serve as tests for theoretical
statements only if the observations are dependent on theories completely independent of those being tested. The method of
outgroup comparison cannot be saved by
reference to the "method of reciprocal illumination" (Hennig, 1966, p. 21) or by
comments that systematists must start with
the existing classification in their efforts to
improve it. Neither of these assertions rids
the method of out-group comparison of its
circularity.
The valid tests of character hypotheses,
the methods to estimate the degree of confidence in the results of individual tests,
and the use of character hypotheses to test
the two types of group hypotheses will be
covered briefly; the interested reader is referred to my earlier paper (Bock, 1977a,
pp. 880-891) for details. I am concerned,
herein, mainly with the role of F-A A in
these tests.
Classificatory hypotheses about groups
are tested by hypotheses about homologies
of characters which must include conditional phrases describing the nature of the
16
WALTER J. BOCK
homology. These conditional phrases are
arranged into hierarchies, both horizontally (describing sister-group relationships)
and vertically (describing ancestral-descendent relationships). The only valid test of
homology is shared similarities of all types
between the presumed homologous features; however, this test is a poor one with
low resolving power. Confidence in the
correctness of the test of individual homologues is judged by considering factors
such as the complexity of the feature, the
relationship between form-functional
properties of the feature and the selection
forces governing its evolution (to ascertain
the probability of independent origins,
convergence, multiple evolutionary pathways, correlation between morphological
and adaptive convergence), etc. These
considerations and others (e.g., Reiger and
Tyler, 1979) are based largely on an assessment of the adaptive evolution of the
homologous feature for which a F-A A of
the feature is essential. Thus F-A A do not
have a role in the valid testing of homologues, but are essential in the estimation
of the degree of confidence in the results
of individual tests.
Because of the low resolving power of
the empirical method used to test hypotheses about homology, a large number of
homologues are needed to test classificatory hypotheses about groups. If a high
degree of confidence in the homology of
features can be established by additional
study, then the degree of confidence in the
classificatory hypotheses about the groups
is increased.
Phylogenetic hypotheses about groups,
be they ancestral-descendent or sistergroup relationships, are tested by a sequence of character hypotheses involving
the taxonomic properties of homology and
plesiomorphy-apomorphy. These character hypotheses must be tested at a series of
steps; namely:
(a) The first step is to formulate and test
hypotheses about homologues, including
the establishment of the degree of confidence in the correctness of the test, as has
just been outlined. Many workers believe
that this step can be omitted, but it is the
essential initial step in any character analysis.
(b) Second is the arrangement of homologues into transformation series which reflect the successive chronological steps in
the evolution of the feature. This step ma]l"
have been done, at least in part, in the establishment of the hierarchies of conditional phrases of the homologues. Transformation series are not established by
chance, but must reflect what is believed to
be the sequential steps in the evolution of
the feature. Transformation series are
tested in the light of hypotheses about
mechanisms of phyletic evolutionary
change (a N-D E) and with concepts about
the evolution of particular types of structures (e.g., how skeletal muscles evolve),
and finally against the empirical observations used to test these hypotheses.
Because individual features do not
evolve independently of organisms in
which they are found and because it is necessary to show that the organisms are integrated functional wholes, it is frequently
necessary to consider the evolution of other features when establishing and testing
a particular transformation series. Or it
may be necessary to compare the transformation series of individual features with a
phylogenetic sequence of organisms as advocated by Gutmann (see above).
Because transformation series describe
the presumed evolutionary histories of
features, it is not possible to establish or to
test them without a clear understanding of
the mechanisms of adaptive evolutionary
change of these features. And this understanding is not possible in the absence of
thorough functional and adaptive analyses. I know of no methods by which hypotheses about transformation series can
be established and tested in the absence of
F-A A.
(c) Third is the establishment of the
evolutionary polarity of the transformation series—which condition of the feature
is primitive (plesiomorphous) and which
conditions are advanced (apomorphous).
Two valid tests are known to me.
(1) Use of information about the relative stratigraphic positions of fossils pos-
EVOLUTIONARY CLASSIFICATION
sessing conditions of the feature arranged
in the transformation series. Fossils found
earlier in the fossil record would possess the
more primitive condition than those found
later in the record. Recall that the trans"Wormation series has already been established and the fossil record is used only to
test the polarity of the series; the fossil record is not used to establish the transformation series. Confidence in the resolving
power of this test depends on the age span
between the fossils showing primitive and
advanced conditions compared to the total
age of the group and on the completeness
of the fossil record. Rejection by many taxonomists of the fossil record as a valid test
for the polarity of transformation series
stems from the study of Schaeffer et at.
(1972) in which invalid tests were confused
with poor tests' and the use of the fossil
record to establish transformation series
was confused with the use of the fossil record to test the polarity of previously established series.
(2) Use of information on the mechanisms of evolutionary change, on the evolution of particular types of structures and
on the empirical observations used to test
these secondary hypotheses as discussed
above in the testing of transformation series. Again, for the reasons given above, I
know of no way that this type of test of the
polarity of transformation series can be
made in the absence of thorough F-A A.
Special mention must be made here of
the use of postulated adaptive changes in
the feature as a test of polarity of transformation series. This is based on the argument that evolutionary change will always be in the direction of the better
adapted state. The direction of change can
be argued strongly provided that a reasonable assessment of environmental factors
and of selection forces can be made. This
has been developed in great detail under
the heading of "die Lesrichtung" by Gutmann and his associates (Gutmann, 1966,
1969, 1977; Peters and Gutmann, 1971,
1973; Gutmann and Peters, 1973a, b) and
used in analysis of a number of vertebrate
and invertebrate groups. These ideas apply to the formation of transformation se-
17
ries, both of individual features and of
whole organisms, to the establishment of
the polarity of these series and to the determination of nonreversibility of change
in these series (see below) as well as to the
testing of these character hypotheses. Unfortunately these powerful methods are little known and used by English-speaking
taxonomists.
(d) Fourth is the establishment of nonreversibility of change in the transformation series. This step can be included in
the previous ones ("b" and "c") by the establishment of more complex series; however, this possibility is rarely included. Establishment of nonreversibility of change
is essential because it permits a distinction
between those transformation series in
which reversal of evolutionary change may
occur (resulting in "secondarily primitive"
conditions) and those in which reversal
does not occur. Distinction between these
two categories of transformation series is
important because those with nonreversible change provide stronger tests for phylogenetic hypotheses about groups. Testing of hypotheses about nonreversible
change can be made only with the use of
information on mechanisms of evolutionary change and on the evolution of particular types of structures, and the empirical
observations on which these secondary hypotheses are tested for the same reasons
presented above. Again, I know of no way
that hypotheses about nonreversibility of
change in transformation series can be
tested without thorough F-A A.
(e) Last is the separation of the class of
shared apomorphous features into the
subclass of synapomorphs and autapomorphs (both of which are homologous
apomorphy, Bock, 1977a, p. 888) and the
subclass of convergent features (nonhomologous apomorphs). If the first step of
ascertaining and testing homologues has
been done thoroughly, then little or nothing has to be done at this last step. In any
case, the methods and tests used, including
estimating the degree of confidence in the
correctness of individual tests, are exactly
the same as those used in determining and
testing for homologies (step a). Again F-A
18
WALTER J. BOCK
A are an important element in estimating
the degree of confidence in the correctness
of individual tests.
Contrary to the claims of many cladists
(e.g., Wiley, 1975), synapomorphs are not
identical to homologues and the criteria
for testing synapomorphs are not the same
as those for testing homologues. I need
only point out that tests for synapomorphs
must include all those for the establishment of transformation series and for
their polarities, none of which appear in
the tests for homologous features. And the
concept of homology is far broader than
that of synapomorphy because symplesiomorphs are perfectly good homologues.
One need only read Hennig (1965, 1966,
pp. 73-74, 146-147) carefully to realize
that when he speaks of the need to analyze
further the "concept of similarity" and
shows how this can be done, Hennig
equates the concepts of symplesiomorphy
and synapomorphy with subdivisions of the
concept of homology.
CONCLUSIONS
The classification and the phylogeny of
any group of organisms are theory-laden
historical narrative explanations in that
they are based on nomological-deductive
explanations drawn from the underlying
theory of organic evolution. In formulating a classification or a phylogeny, the systematist is attempting to reconstruct the
past evolutionary history of the group and
to represent it in a particular formal system—a Linnean hierarchy of taxa or a dichotomous dendrogram of phyla. Both
types of historical narrative explanations
are theoretical scientific statements and
must be tested against empirical observations. These tests may be falsifications or
confirmations, but they always ultimately
employ nomological deductive explanations (e.g., the mechanisms of evolution
and the evolution of particular types of
structures), but not other historical narrative explanations, in the argument chain
leading to the empirical observations.
Tests of classificatory and of phylogenetic
hypotheses about groups may be placed
together under the heading of character
analysis which includes the formulation of
hypotheses about taxonomic properties of
characters (e.g., homology, synapomorphy, etc.), the testing of these character
hypotheses against empirical observations,
and their use in testing the group hypotheses. Because the taxonomic properties of
characters express information about their
evolutionary history, the complete set of
valid tests of these character hypotheses
and of the methods used to establish the
degree of confidence in the results (i.e., the
resolving power) of the individual tests is
absolutely dependent on thorough functional and adaptive analyses in addition to
proper morphological description and
comparison. I know of no way to test successfully ideas on the evolution of particular types of structures (e.g., muscles, articulations, vertebrate cranial kinesis, the
coelom as a hydrostatic skeleton, etc.) in
the absence of functional and adaptive
analyses. It may not be possible to undertake functional and adaptive analyses for
all taxonomic features, but the attempt
should always be made. As a general rule,
the more thorough these functional and
adaptive analyses have been, the better are
the tests of the character hypotheses, the
greater is the degree of confidence in the
results of individual tests and the greater
is the degree of confidence in the tests of
the classificatory and the phylogenetic hypotheses about groups. Regardless of the
completeness of all other parts of a systematic investigation, I feel uncomfortable
(i.e., lack confidence) with the conclusions
in the absence of a functional-adaptive
analysis. In conclusion, regardless of the
accepted approach to classification, functional and adaptive analyses of taxonomic
characters are an essential part of all studies of the phylogeny and classification of
biological organisms.
ACKNOWLEDGMENTS
I wish to thank Professors Arthur CapIan, Dominique G. Homberger, Ernest Nagel and Frederick E. Warburton for their
many valuable discussions during preparation of this analysis and for their helpful
criticisms and suggestions on the manuscript. My earlier morphological and taxonomic studies that formed the foundation
EVOLUTIONARY CLASSIFICATION
for this analysis were supported by a series
of grants from the National Science Foundation; preparation of this paper was done
under grant DEB-76-14746 from the NSF.
*
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